The invention relates to a method of generating EUV radiation, in which a mobile medium is injected into a vacuum source space and each time a part of said medium in the source space is irradiated with a pulsed and focused energy-rich laser beam, and in which said medium part is converted into a plasma-emitting EUV radiation, whereafter the medium is passed through a first further vacuum space in series with the source space, said vacuum space being maintained at a lower vacuum degree than the source space.
The invention also relates to a method of manufacturing a device with the aid of this radiation. The invention further relates to an EUV radiation source unit and to a lithographic projection apparatus provided with such a radiation source unit.
A mobile medium is understood to mean a medium which does not have a solid shape but whose shape is determined by the holder accommodating the medium or the guide through which the medium is transported. A vacuum space is understood to mean a space in which a vacuum degree at a pressure of the order of 10.sup.-1 mbar or lower prevails.
A lithographic apparatus is used, inter alia, in the manufacture of integrated electronic circuits or ICs for imaging an IC mask pattern, present in a mask, each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic apparatus may also be used in the manufacture of, for example, liquid crystalline image display panels, integrated, or plenary optical systems, charge-coupled detectors (CCDs) or magnetic heads.
Since it is desired to accommodate an ever increasing number of electronic components in an IC, increasingly smaller details, or line widths, of IC patterns must be imaged. Consequently, increasingly stricter requirements are imposed on the imaging quality and the resolving power of the projection system in the apparatus, which projection system is generally a lens system in the current lithographic apparatuses. The resolving power, which is a measure of the smallest detail which can still be imaged satisfactorily, is proportional to .lambda./NA, in which .lambda. is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolving power, the numerical aperture may, in principle, be enlarged and/or the wavelength may be reduced. An increase of the numerical aperture, which is currently already fairly large, is no longer very well possible in practice because the depth of focus of the projection system, which is proportional to .lambda./NA.sup.2, becomes too small and the correction for the required image field becomes too difficult.
The requirements to be imposed on the projection system may be mitigated or, when maintaining these requirements, the resolving power may be increased if, instead of a stepping lithographic apparatus, a step-and-scan lithographic apparatus is used. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one run and imaged as one whole on an IC area of the substrate. After a first IC area is illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate are provided with the mask pattern. In a step-and-scan apparatus, each time only a rectangular or annular segment-shaped area of the mask pattern, and hence a corresponding sub-area of a substrate IC area is illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, taking the magnification of the projection system into account. Each time, a subsequent area of the mask pattern is imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged in this way on an IC area, the substrate holder performs a step, i.e. the start of a subsequent IC area is introduced into the projection beam and the mask is set to, for example, the starting position, whereafter said next IC area is scan-illuminated via the mask pattern.
If even smaller details are to be satisfactorily imaged with a step-and-scan lithographic apparatus, the only possibility is to reduce the wavelength of the projection beam. In the current step-and-scan apparatuses, already deep UV (DUV) radiation is used, i.e. radiation having a wavelength of the order of several hundred nanometers, for example, 248 nm or 193 nm from, for example, an excimer laser. Another possibility is the use of extreme UV (EUV) radiation, also referred to as soft X-ray radiation, having a wavelength in the range of several nm to several tens of nm. Extremely small details, of the order of 0.1 .mu.m or smaller, can be satisfactorily imaged with such a radiation.
Since no suitable lens material is available for EUV radiation, a mirror projection system instead of a hitherto conventional lens projection system must be used for imaging the mask pattern on the substrate. For forming a suitable illumination beam of the radiation from the EUV radiation source, mirrors are also used in the illumination system. The article "Front-end design issues in soft X-ray lithography" in Applied Optics, Vol. 23, No. 34, Jan, 12, 1993, pp. 7050-56 describes a lithographic apparatus in which EUV radiation is used and whose illumination system comprises three mirrors and the imaging, or projection, system comprises four mirrors.
As described in the article "Debris-free soft X-ray generation using a liquid droplet laser-plasma target" in Applications of Laser Plasma Radiation II, SPIE 2523, 1995, pp. 88-93, EUV radiation can be generated by focusing a laser beam on water droplets. The required stable flux of individual micro water droplets can be obtained by means of a capillary tube which is caused to vibrate by a piezoelectric driver. Each water droplet impinged upon by the laser beam is consecutively converted by the high temperature into a plasma which emits EUV radiation.
In EUV lithographic apparatuses, it is a great problem to illuminate the substrate at a sufficiently high intensity. A first cause of this problem, which applies to all EUV apparatuses, is that the mirrors used are considerably less than 100% reflecting. Each of these mirrors has a multilayer structure whose composition is adapted as satisfactorily as possible to the wavelength of the projection beam used. Examples of such multilayer structures are described in U.S. Pat. No. 5,153,898. A multilayer structure which is often mentioned in literature is the structure consisting of silicon layers alternating with molybdenum layers. For radiation from a plasma source, mirrors provided with such a multilayer structure theoretically have a reflection of the order of 73% to 75%, but in practice the reflection is currently not larger than 65%. When using said number of seven mirrors, each with a reflection of 68%, only 6.7% of the radiation emitted by the source reaches the substrate. For a lithographic apparatus, this means in practice that the illumination time should be relatively long so as to obtain the desired quantity of radiation energy on a substrate, and for a scanning apparatus, particularly the scanning rate should be relatively small. However, it is essential for these apparatuses that the scanning rate is as high as possible and the illumination time is as short as possible so that the throughput, i.e. the number of substrates which may be used per unit of time, is as high as possible. This can only be achieved with an EUV radiation source supplying a sufficient intensity. A second cause of the problem is related to the fact that the radiation path for the EUV radiation must extend in a vacuum, i.e. in a space with a pressure of the order of 10.sup.-2 mbar and preferably lower, because otherwise too much EUV radiation is absorbed. In a water plasma source, there is the problem that the water droplets moving through the source space give off water vapor whose vapor pressure at room temperature is approximately 23 mbar. Without any further measures, it is therefore impossible to comply with a vacuum requirement of 10.sup.-2 mbar and preferably considerably lower, for example, 10.sup.-6. Moreover, the water vapor may get through apertures in the wall of the source space, which apertures are intended for causing the laser beam to enter and exit from the source space and for causing the generated EUV radiation to exit from the source space so that the water vapor may deposit on the mirrors of the illumination system and the projection system and attack these mirrors, thus reducing their reflection. Similar problems occur when using other liquids or gases such as clustered xenon with which an EUV-emitting plasma can be formed.
The article "Laser produced oxygen plasmas" in Proceedings of the Second International Symposium on Heat and Mass Transfer under Plasma conditions, 1999 proposes to solve the problem of the water droplets and the water vapor by providing an extra vacuum space behind the source space in the direction of movement of the medium and to interconnect the two spaces via a narrow opening. The extra vacuum space is connected to its own vacuum pump which maintains a pressure in this space which is higher than the pressure in the source space. The object of this so-called differential pumping is that the water droplets injected into the source space and expanding and partially changing over to water vapor enter the space where they are converted into water vapor. Also the water vapor formed in the source space must enter the extra vacuum space so that all the water injected into the system is removed as water vapor by the pump of the further vacuum space. However, it has been found that this solution still presents problems.